The present invention relates in general to accelerometers and in particular to a new and useful fiber optic accelerometer which uses an optical fiber that is clamped between two corrugated plates to measure vibration. The present invention also relates to an electronic circuit used for measuring the light signals from the optical fibers and converting them to electrical signals which are proportional to the acceleration to be measured by the accelerometer. One of the plates acts as an inertial mass which is carried by a flexible diaphragm and which moves under the influence of vibrational forces to apply a bending force on the optical fiber. Light passing through the optical fiber varies in accordance with the bending of the fiber. This variation in the light is measured at a remote location, as an indication of the acceleration.
Accelerometers are devices which are used to measure shock and vibration. They consist of an inertial mass which is attached to a casing by a spring. When the casing is attached to a structure whose vibration (acceleration) is to be measured, the vibration of the casing is transmitted to the inertial mass. The inertial force, which is proportional to the vibrational level acts on the spring. In the case of piezoelectric accelerometers, a piezoelectric crystal acts as a spring. The inertial mass acts on the crystal and induces a charge which is proportional to the force. The charge, which gives a measurement of the vibrational level, is measured. In the case of the piezoresistive or strain gage accelerometer, the deformation of the spring caused by inertial force is measured using strain gages.
Both types of accelerometers can be used for room temperature applications. At high temperatures, however, only the piezoelectric accelerometers can be used. These, however, show limitations when used in pressurized water reactor (PWR) environments.
Tests have been performed to determine the performance of piezoelectric and other commercially available accelerometers, in hostile PWR environments. In these environments, pressures in excess of 2,500 psi, temperatures of up to about 700.degree. F. and corrosive chemicals can be expected. These tests show that most conventional piezoelectric accelerometers do not survive such conditions. The accelerometers with synthetic crystals failed within a period of a few days. Only one accelerometer passed the qualifying tests. This accelerometer contains a natural crystal as opposed to a synthetic one, and costs about $5,000.
Accelerometers which use fiber optic systems are known. One example is found in U.S. Pat. No. 4,408,495 to Couch et al. Couch et al measures the displacement of a vibrating object with respect to a fixed reference. No provision is made to measure the absolute acceleration of a vibrating object however. To obtain the absolute acceleration from the displacement with respect to the fixed reference requires that the relative displacement be differentiated twice. This is done with sophisticated electronics and is inherently a "noisy" operation. Any noise in the displacement signal is greatly amplified when the signal is differentiated twice to reach the acceleration value.
In requiring a fixed reference, the application of the Couch et al patent is also limited since a fixed reference is usually not available to measure vibration. In cases where the reference surface is undergoing vibration, the absolute vibration of the reference must be known.
U.S. Pat. No. 4,552,026 to Knudsen et al describes a sensor for a vortex shedding flowmeter which utilizes an optical fiber that is clamped between inner and outer tooth rings. Relative rotation between the rings as a result of the passage of vortices, causes a microbending of the optical fiber. Light passing through the optical fiber is thus varied and this variation is measured to measure the flow rate as a function of the shed vortices.
Other U.S. Patents which utilize optical fibers in accelerometers are U.S. Pat. No. 4,353,259 to Schneider, Jr.; U.S. Pat. No. 4,376,390 to Rines; U.S. Pat. No. 4,403,144 to Strahan et al and U.S. Pat. No. 4,525,626 to Kush et al.
Other devices which are interesting for their disclosure of optical mechanisms for use in measuring different parameters are U.S. Pat. No. 4,471,659 to Udd et al; U.S. Pat. No. 4,405,198 to Taylor; U.S. Pat. No. 4,379,226 to Sichling et al; U.S. Pat. No. 4,414,471 to Rines; U.S. Pat. No. 4,472,022 to Bearcroft et al and U.S. Pat. No. 4,214,485 to Berger et al.